1. Field of the Invention
The present invention relates generally to non-linear optical fibers and light sources, and particularly to a light source based on efficiently producing light via third-harmonic generation in an optical fiber.
2. Technical Background
Non-linear optical materials have the property that the electric polarization responds non-linearly to an incident optical field. The non-linearity of an optical material is typically expressed as a power series expansion of the total applied optical field, with susceptibility coefficients χ(1), χ(2) and χ(3) representing the linear and non-linear second-order and third-order susceptibilities, respectively.
Third-order harmonic (TH) generation of light (simply called “TH generation”) is a nonlinear optical process that exploits the third-order susceptibility χ(3) of an optical material to generate light with triple the frequency of the incident (pump) light. In many materials, the third-order non-linear susceptibility, however, is typically orders of magnitude smaller than the second-order susceptibility, so that corresponding third-order effects have much smaller conversion efficiencies relative to second-order effects (e.g., second-harmonic generation of light).
While direct TH generation is theoretically possible in a number of different materials, light sources based on direct TH generation have very low conversion efficiencies and so are generally not used in practice. Presently, virtually all frequency-tripling applications avoid direct TH-generation in a single medium, and use a two-stage process of second-harmonic generation in a first non-linear crystal with a strong χ(2) coefficient, followed by frequency summing in a second non-linear crystal with a strong χ(2) coefficient.
For direct TH generation to be useful in practice, a conversion efficiency of 1% or greater is needed. However, when the fundamental (pump) wave travels through the material at a different velocity than does the resulting harmonic wave due to the normal dispersion of the optical material, the proper phase between the fundamental and harmonic waves is not maintained. The result is that the third-harmonic waves and the pump wave destructively interfere, resulting in poor conversion efficiency. Accordingly, one technique used to increase the conversion efficiency in non-linear optical processes is called “phase-matching.” Phase-matching involves maintaining the proper phase between the pump and harmonic wave so that the harmonic waves and the pump wave constructively interfere.
TH generation is possible in optical fibers. The third-order susceptibility coefficient χ(3) is generally stronger than the second-order susceptibility χ(2) in optical fibers due to the lack of crystal symmetry of the fiber. However, the glass material dispersion property of optical fibers prevents phase matching between the fundamental modes of the pump light and the TH-generated light. Further, the overlap between the pump and harmonic optical fields in conventional optical fibers is typically very small, making the conversion efficiency so low (<1%) as to be useless for commercial applications. The practical effect of this shortcoming is that there is no robust commercial light source based on direct TH generation that can be used to generate light that can be used for a variety of applications ranging from medical treatment to materials processing to laser projection displays.
Microstructured fibers have been proposed for TH generation in guided modes, but the conversion efficiency is well below 1%. Glass micro-fibers have also been proposed for TH generation. However, while good efficiency is theoretically possible, it is very hard in practice to make microstructured fibers with sub-wavelength diameter. Further, it is very difficult to optically couple to such small fibers.
Accordingly, what is needed is an optical-fiber-based light source that uses direct TH-generation at sufficiently high conversion efficiencies so that the light source can be used for commercial applications.